Well, my Pioneer SX-950 is an old receiver, about 30 years, and the protective sleeve over one of the resistors on the board that the FETs are hooked up on has turned brown, just in the center of it though. Is this from heat damage? If it is, since the resistor is heat damaged, and the resistance changed, could the other components on that board have gone out of phase with is over the years, and if I changed it would it majorly screw up my sound?
Components will age and drift away from their original values over time. The brown spot would indicate some heat which is always bad for components. If you can find someone to look at the receiver, you may want to get an estimate of what needs to be replaced. At 30 years of age it may be best to use the unit until it fails and then consider you got your money's worth.